feat: Implement nausea symptom system and related features

- Added SymptomManager autoload for managing symptom intensities.
- Introduced nausea tilt effect for camera roll based on nausea intensity.
- Created color desaturation shader and overlay for visual feedback.
- Developed TreatmentManager to handle nausea ramping during IV treatment.
- Added hospital room scene with first-person controller and interactions.
- Implemented sit-down interaction for the treatment chair.
- Created IV insertion sequence with animations and completion signal.
- Added player controller with movement and interaction capabilities.
- Integrated nausea effects into the player experience with smooth transitions.

scripts/symptoms/nausea_desaturation.gd

@@ -0,0 +1,51 @@
+extends ColorRect
+## Drives the desaturation shader's `desaturation` uniform from nausea intensity.
+##
+## Nausea ramps desaturation from 0 (no effect) up to full grayscale at
+## `max_at_intensity`. The uniform is smoothly faded toward its target.
+## Attach to a full-screen ColorRect that uses `desaturation.gdshader`.
+
+## Nausea intensity at (and above) which the screen is fully desaturated.
+@export var max_at_intensity: float = 80.0
+## How fast the shader uniform follows its target, per second.
+@export var fade_speed: float = 2.0
+
+var _target: float = 0.0
+var _current: float = 0.0
+
+
+func _ready() -> void:
+	# Cover the whole viewport and never block input.
+	mouse_filter = Control.MOUSE_FILTER_IGNORE
+	set_anchors_preset(Control.PRESET_FULL_RECT)
+
+	var sm: Node = null
+	if is_inside_tree():
+		sm = get_tree().root.get_node_or_null("SymptomManager")
+	if sm == null:
+		sm = SymptomManager
+	if sm:
+		sm.nausea_intensity_changed.connect(_on_nausea_changed)
+		_set_target_from_intensity(sm.get_intensity(sm.NAUSEA))
+		_current = _target
+		_apply()
+
+
+func _on_nausea_changed(value: float) -> void:
+	_set_target_from_intensity(value)
+
+
+func _set_target_from_intensity(intensity: float) -> void:
+	_target = clampf(intensity / max_at_intensity, 0.0, 1.0)
+
+
+func _process(delta: float) -> void:
+	if is_equal_approx(_current, _target):
+		return
+	_current = move_toward(_current, _target, fade_speed * delta)
+	_apply()
+
+
+func _apply() -> void:
+	if material is ShaderMaterial:
+		(material as ShaderMaterial).set_shader_parameter("desaturation", _current)